Many contemporary control systems for controlling multi-cylinder internal combustion engines rely on a direct measurement of exhaust gas chemistry at the engine's output to finely tune a combustion process. The direct measurement is important because by using this measurement in a control system, emissions can be reduced to a level necessary to comply with legislated emissions standards.
Of the various exhaust gas constituents oxygen is the most commonly measured gas in production vehicles. By measuring oxygen concentration an air-fuel ratio can be determined. Air-fuel ratio is useful for controlling engine emissions. For determination of oxygen concentration and thus air-fuel ratio, most modern control systems rely on a particular type of exhaust gas chemistry sensor known as an EGO, or exhaust gas oxygen sensor. This sensor is also commonly known as an O.sub.2 or a .lambda. (lambda) sensor. This sensor is typically positioned in an exhaust gas stream between the engine's exhaust manifold and a catalytic converter. The oxygen concentration in the exhaust gas stream is indicative of the exhausted air-fuel ratio. This two state sensor senses that the engine's exhaust gas stream is either rich or lean about a point of stoichiometry which corresponds to an air-fuel ratio of about 14.6:1 for gasoline fuel. When the exhaust gas stream has an air-fuel ratio of 14.6:1 then stoichiometric combustion is taking place in the engine and emissions, particularly carbon monoxide, are reduced significantly by the catalytic converter.
In a multi-cylinder engine the exhaust gas stream represents a combined output from all of the engine's cylinders. The engine control system is responsible for controlling the supply of fuel such that with the current air supply the measured air-fuel ratio of the exhaust gas stream observes stoichiometric combustion. It is well established that differences in engine intake port geometry and fuel system component tolerances will cause a significant difference in individual cylinder contribution to the air-fuel ratio of the exhaust gas stream.
As legislation continues to require tougher standards these measurement and control systems must get better. One approach has been to control the fuel injected on a cylinder-by-cylinder basis. This requires a sensory system that can distinguish individual cylinder air-fuel ratios in the exhaust gas stream. This individual cylinder control of air-fuel ratio allows for compensation of the aforementioned engine component part intolerance and geometric factors. As a result of applying this approach, carbon monoxide and other emissions can be reduced in support of the legislated emissions standards.
To implement a control system for controlling individual cylinder air-fuel ratios, some have suggested the use of a singular exhaust gas sensor. Some systems use the traditional EGO sensor and some use a proportional sensor. These proportional sensors provide a signal representative of a measured air-fuel ratio that is proportional to, or linearly dependent on, the oxygen content of the measured exhaust gas stream. This is in contrast with more conventional EGO sensors which only provide a discrete two-state output indicating either a rich or lean measurement as described above. Although the measurement of individual cylinders with a traditional EGO sensor shows an improvement over contemporary systems, the combination of proportional and individual cylinder measurement can provide the additional measurability necessary to meet emissions standards. Proportional air-fuel ratio sensors are commonly known as UEGO, or universal exhaust gas oxygen sensors. Of course, to measure individual cylinders individual UEGO sensors could be positioned at each exhaust port of a multi-cylinder engine. However, this approach is cost, weight, and space prohibitive. Further, it is not very reliable because the added UEGO sensors and interface support substantially reduces the system's reliability.
Applying a singular oxygen sensor, in a combined exhaust gas stream for measuring individual cylinder air-fuel ratios presents many difficult technical challenges to a measurement system. Prior art systems showing a singular oxygen sensor in a combined exhaust gas stream have not addressed all of these technical challenges. These technical challenges include static and dynamic temporal and spacial effects characteristic of a multi-cylinder internal combustion engine and its exhaust system. Also, oxygen sensors have response speed limitations that limit the useful frequency response of the sensor.
Regarding the response speed limitations of oxygen sensors, both UEGO and EGO sensors have a distortion characteristic dependent on the particular sensor's frequency response. A distortion manifests itself as signal attenuation and phase delay as higher frequency components are presented to the sensor. On typical sensors the useful frequency response is limited to about 11 Hz for the required accuracy. This is not sufficient for accurately measuring the air-fuel ratio of individual exhausting cylinders. For instance, at 2,000 RPM a typical eight cylinder engine will be exhausting at a rate of 67 times per second. Given this rate of change in the measured quantity, the frequency response of the typical oxygen sensor is inadequate to accurately operate in a multi-cylinder architecture.
The temporal considerations can be categorized into system related effects, and exhaust gas transport related effects. Note that the term transport, as it applies here, refers to an interconnection between the cylinders' exhausting ports and the oxygen sensor. This may include an exhaust manifold for combining multiple cylinders into a singular pipe.
Regarding system related temporal effects, different engines have different exhausting orders. This means that while one sequence may be 1-3-4-2, another may be 1-2-4-3 in a four cylinder example. Because of this, the particular exhausting cylinder must be identified at the oxygen sensor output, when evaluating individual cylinder air-fuel ratio. This is because when a particular cylinder is exhausting, the output of the sensor will respond to the exhaust gas stream chemistry associated with that particular cylinder. Other temporal considerations discussed below will further complicate this synchronization of exhausting order process. Another system related temporal effect is that of next-cylinder blow-off. When a particular exhaust port opens, the gaseous mixture in the corresponding cylinder's combustion chamber flows out into the exhaust manifold at a significant flow rate. Because the incumbent flow rate is relatively low in the exhaust manifold, a high rate of change of flow rate exists and the gaseous mixture rapidly displaces, or blows-off the incumbent gaseous mixture MATRIX 1
proximate the oxygen sensor. Because of this rapid change in flow rate, and the resulting rapid change of chemistry between the incumbent gaseous mixture and the next-cylinder's gaseous mixture, the oxygen sensor's output may change significantly. If this blow-off effect is not properly accounted for, a significant error may occur when interpreting the air-fuel ratio of individual exhausting cylinders. Next, exhaust gas transport temporally related effects will be detailed.
Multi-cylinder engines typically have an exhaust manifold structure with geometry dependent on a particular position of a cylinder. A difference in a length of a transport path of the exhaust gas stream is inherent if one cylinder's exhaust port is positioned physically closer to the downstream oxygen sensor than another. The different exhaust runner lengths, and the corresponding difference in the length of the transport paths, will cause a difference in propagation delay of the exhaust gas stream between exhausting cylinders from when an exhaust port opens to when the oxygen sensor senses the change in oxygen content in the exhaust gas stream. Additionally, a flow impedance presented to a particular cylinder's exhaust gas stream can be dynamically affected by engine load. This will cause a dynamic difference on propagation delay. Existing measurement systems do not compensate for these exhaust gas transport temporally related effects.
Transport spacial characteristics affect static flow impedance and may have certain resonant characteristics affecting the behavior of the exhaust gas stream for particular cylinders of a multi-cylinder engine. These spacial characteristics are caused by the particular geometry of the exhaust system. These transport spacial characteristics may result, for instance, in spectral attenuation and/or reflection of the exhaust gas stream.
Additionally, oxygen sensors typically exhibit different response characteristics depending on whether the exhaust gas is rich or lean. Unless this is accounted for, a computed air-fuel ratio will have significant error.
What is needed is an improved sensory system for measuring gas chemistry of a combined gas stream exhausted from a multi-cylinder internal combustion engine. The improved sensory system must take into consideration the static and dynamic temporal and spacial effects characteristic of a multi-cylinder engine exhaust system, the characteristic frequency response limitations of a gas chemistry sensor and the difference in sensory response to rich and lean exhaust gases.